The mechanism of action of hormones is studied in epithelia formed in culture. To understand the responses better, the cell biology of the epithelia is studied as well as transport specific events. The amiloride sensitive sodium channel in the apical plasma membrane of A6 epithelia is studied in vesicles enriched in apical membranes. Amiloride sensitive sodium flux is increased in vesicles prepared from epithelia stimulated with vasopressin. The sodium transport response of intact epithelia to vasopressin, cholera toxin, and cAMP is enhanced following the incorporation of exogenous gangliosides into the apical plasma membrane.